Trivial return types of generated get methods should be by value instead of const reference

[tmadlener]

Currently all return types of the generated get methods for the user facing handles return by const&:

podio/python/templates/macros/declarations.jinja2

Lines 11 to 12 in 858c0ff

	/// Access the {{ member.docstring }}
	const {{ member.full_type }}& {{ member.getter_name(get_syntax) }}() const;

This can have negative performance implications and it might lead to subtle differences with ExtraCode where the value might be returned by value. Mixing podio generated get functions and others that return by value can be very hard in generic code this way.